This invention relates to the control of current in the inductor or coil of a superconducting magnet which serves as a store of magnetic energy to be used, by way of example, as a back-up source of electric power in an uninterruptable power supply (UPS). More particularly, the invention relates to the use of gating circuitry for direction of current flow through the inductor and for control of average current and voltage at the inductor, wherein a charging and a discharging of energy of the magnet is accomplished via a series of voltage-step-up and step-down chopper circuits with outputting of electric power by successive capacitive energy storage cells.
A superconducting magnet comprises a coil or inductor of a superconducting material, and can serve as a useful storage of energy in the form of a magnetic field produced by a flow of electric current in the inductor. An example of an energy storage system incorporating a superconducting magnet is a UPS, wherein electric power is applied to a load by a public utility, the electric power company, and wherein the electric power for the load is withdrawn from the stored magnetic energy of the superconducting magnet during a malfunction in the supply of the utility power. Typically, in the construction of a superconducting magnetic energy storage system (SMES), various electrical circuits are employed for charging the magnet with current, maintaining a desired level of current in the magnet and extracting current from the magnet when back-up power is required for the load.
Examples of circuitry employed for control of current in a superconducting magnet are provided by the U.S. Pat. Nos. 4,962,354 and 5,194,803 of Visser et al and 5,159,261 of Kim et al. These patents disclose the interconnection of a superconducting magnet with an energy storage cell by means of switching circuitry, and wherein the output of the energy storage cell is to be applied to a load. A problem arises in that the circuits of the foregoing patents are unduly complex and, therefore, may not be optimum in terms of efficient use of the electrical components.